Cryogenic purification with heat uptake

ABSTRACT

Process for purifying a gaseous feed stream using an adsorption unit comprising at least two adsorbers, a cryogenic distillation unit, an exchanger and a compressor operating at a temperature less than or equal to −50° C., in which the heat necessary for the regeneration of the adsorbers is derived, at least partly, from at least one portion of the heat generated by the compressor, during the compression of a fluid.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 of International PCT Application PCT/FR2016/051567, filed Jun. 12, 2015, which claims priority to French Patent Application No. 1455985, filed Jun. 26, 2014, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present invention relates to a process for purifying a feed gas stream using an adsorption unit and a cryogenic distillation unit.

Adsorption is a phenomenon generally promoted by a low temperature. For example, for an ASU (Air Separation Unit), the stopping of CO₂ on a molecular sieve is up to 5 times greater at −100° C. than at 20° C., and around 3 times greater for the stopping of propane.

The regeneration requires make-up heat which disturbs the refrigeration balance of the equipment, if the adsorption took place at a negative temperature. Its energy cost may be even greater since the temperature is low.

In the processes according to the prior art, the adsorption is carried out at a positive temperature and the heat for regenerating (the excess heat) is discharged to the atmosphere without impacting the refrigeration balance of the cryogenic portion.

Starting from there, one problem that is faced is that of providing a cryogenic purification in a cryogenic separation process that already knows how to manage, when it comes to the refrigeration balance, a heat gain at least equal to that needed for the regeneration of the adsorbers.

SUMMARY

One solution of the present invention is a process for purifying a feed gas stream using an adsorption unit comprising at least 2 adsorbers, a cryogenic distillation unit, an exchanger and a compressor operating at a temperature below or equal to −50° C., wherein the heat needed for the regeneration of the adsorbers is derived, at least in part, from at least one portion of the heat generated by the compressor, during the compression of a fluid.

Depending on the case, the process according to the invention may have one or more of the following features:

-   -   said process comprises an adsorption step implemented by the         adsorption unit, with the adsorption step performed at a         negative temperature;     -   said process comprises, according to a first alternative, the         following successive steps (FIG. 1):     -   a) the feed gas stream 1 is cooled in the exchanger 2 to a         temperature below −50° C., preferably below −100° C.;     -   b) the cooled gas stream 3 is sent to the adsorption unit 4         where at least one impurity X is at least partly adsorbed so as         to recover a gas stream 5 depleted in impurity X;     -   c) the gas stream depleted in impurity X 5 is introduced into         the exchanger 2 in order to be cooled to a temperature below         −50° C., preferably below −150° C.;     -   d) the gas stream 5 depleted in impurity X and cooled is sent to         the cryogenic distillation unit 7 where it is separated into at         least 2 streams 8 and 9;     -   e) a portion of the stream 9 is introduced into the exchanger in         order to be reheated to a temperature above −150° C., preferably         above −100° C., more preferentially above −50° C., ideally to a         temperature close to that of the feed gas stream 1 at the end of         step a) before being compressed in the compressor 10 with a         compression ratio greater than 1.2;     -   f) the compressed stream 9 is sent to the adsorption unit 4 in         order to regenerate one of the two adsorbers;         with the compression in step e) leading to a temperature         increase of the stream 9 of at least 20° C. and thus providing         the heat input needed for the regeneration of at least one of         the adsorbers;     -   said process comprises, according to a second alternative, the         following successive steps (FIG. 2):     -   a) the feed gas stream 1 is cooled in the exchanger 2 to a         temperature below −50° C., preferably below −100° C.;     -   b) the cooled gas stream 3 is sent to the adsorption unit 4         where at least one impurity X is at least partly adsorbed so as         to recover a first stream depleted in impurity X 5;     -   c) the gas stream 5 depleted in impurity X is compressed in the         compressor 10 with a compression ratio greater than 1.2 before         being cooled in the exchanger 2 to a temperature below −50° C.,         preferably below −150° C.;     -   d) the gas stream 5 depleted in impurity X, compressed and         cooled is sent to the cryogenic distillation unit 7 where it is         separated into at least 2 streams 8 and 9;     -   e) a portion of the stream 9 is introduced into the exchanger in         order to be reheated to a temperature above −150° C., preferably         above −100° C., more preferentially above −50° C., ideally to a         temperature close to that of the feed gas stream 5 at the end of         the compression of step c);     -   f) the reheated stream 9 is sent to the adsorption unit 4 in         order to regenerate at least one of the two adsorbers;         with the compression in step c) leading to a temperature         increase of the gas stream 5 depleted in impurity X of at least         20° C. and thus providing, indirectly via the exchanger 2, the         heat input needed for the reheating of a portion of the stream 9         and therefore for the regeneration of at least one of the two         adsorbers in step f);     -   the adsorbers comprise a monobed, preferably a molecular sieve;     -   the feed gas stream is air and the impurity X is selected from         H₂O, CO₂, N₂O, C_(n)H_(m), NOx;     -   the feed gas stream comprises water and said process comprises,         before step a), a step of pre-purification of the feed gas         stream that makes it possible to eliminate at least one portion         of the water;     -   the pre-purification step is carried out by adsorption at         ambient temperature;     -   the adsorption of the pre-purification step is carried out on a         monobed of alumina, silica gel or molecular sieve type.

The invention will be illustrated on an ASU with a cold compressor. The cold compressor introduces into the cold box a thermal gain that reheats the compressed gas. The natural refrigeration balance of the equipment makes it possible to manage this thermal gain. A portion of the hot gas will be used directly or indirectly via a heat exchange with another fluid in order to carry out the heating phase of the regeneration. This takes place with no real energy penalty, since it does not disturb (or barely disturbs) the refrigeration balance of the equipment.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

FIG. 1 represents the first alternative of the solution according to the invention.

FIG. 2 represents the second alternative of the solution according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The air 1 is cooled in the exchange line 2 (for example, down to −120° C.), then passes through a bed of adsorbent 4 at low temperature (−120° C.), then is reintroduced (optionally slightly hotter, due to the adsorption) into the exchange line 2 for final cooling before being sent to the distillation portion 7.

A portion of the residual nitrogen 9 is drawn off around −120° C. from the exchange line, then compressed in a cold compressor 10 where it is heated up to a temperature of −80° C. for example, then sent to a bed of adsorbent being regenerated. The heat provided by the compression constitutes the heat input needed for the heating phase of the regeneration. The nitrogen is cooled in the bed of adsorbent 4, and is then sent at a temperature around −120° C. to the exchange line 2 for additional reheating up to ambient temperature.

The adsorption temperature may be preferentially close to the “natural” inlet temperature in the cold booster, that is to say the temperature dictated by the process, as though having a conventional ambient temperature purification.

It is seen that the heating phase of the regeneration does not disturb (or barely disturbs) the refrigeration balance of the equipment, this heating phase being carried out by the natural heat input provided by the cold compression. There is therefore no energy penalty in carrying out a cryogenic purification.

Regarding the cooling phase of the regeneration of the process according to the first alternative, a portion of the residual nitrogen is drawn off around −120° C. from the exchange line, then passes firstly into the bed being regenerated (cooling phase), before being compressed, then sent to the exchange line for additional reheating up to ambient temperature.

It is observed that the heating and cooling phase is carried out at a different pressure requiring an intermediate phase of adapting the bed to the correct pressure.

The air 1 is partially cooled down to −120° C., then passes through a bed of adsorbent 4 before being cold compressed 10 where it is heated up to a temperature of −80° C., then sent back to the hotter exchange line 2 for final cooling before being sent to the distillation portion 7. A portion of the residual nitrogen 9 is reheated in the exchange line 2, up to a temperature close to that of the cold-compressed air, for example −80° C., thus indirectly recovering the heat introduced by the compression of the air. The nitrogen thus reheated to −80° C. carries out the heating phase of the regeneration by passing through a bed of adsorbent 4 where it is cooled down to −120° C., then is sent to the exchange line 2 for additional reheating up to ambient temperature.

The adsorption temperature may be preferentially close to the “natural” inlet temperature in the cold booster, typically around the temperature of the vaporization plateau of the oxygen, for example for the conventional single-machine layouts with cold booster (around −120° C. for oxygen pressurized at 40 bar).

Again, it is seen that the heating phase of the regeneration does not disturb (or barely disturbs) the refrigeration balance of the equipment, this heating phase being carried out by the natural heat input provided by the cold compression, indirectly in this case. There is therefore no energy penalty in carrying out a cryogenic purification.

Regarding the cooling phase of the regeneration of the process according to the second alternative, a portion of the residual nitrogen leaves the exchange line at a temperature close to the inlet of the cold compressor (around −120° C.), passes through the adsorbent bed in order to cool it, then is sent to the exchange line for additional reheating up to ambient temperature. In this case, the heating and cooling phases are carried out at the same pressure.

It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above. 

1-8. (canceled)
 9. A process for purifying a feed gas stream using an adsorption unit comprising at least 2 adsorbers, a cryogenic distillation unit, an exchanger and a compressor operating at a temperature below or equal to −50° C., wherein the heat needed for the regeneration of the adsorbers is derived, at least in part, from at least one portion of the heat generated by the compressor, during the compression of a fluid and said process comprises an adsorption step implemented by the adsorption unit, with the adsorption step performed at a negative temperature.
 10. The purification process as claimed in claim 9, wherein the process comprises the following successive steps: a) cooling the feed gas stream in the exchanger to a temperature below −50° C.; b) sending the cooled gas stream to the adsorption unit wherein at least one impurity X is at least partly adsorbed so as to recover a gas stream depleted in impurity X; c) introducing the gas stream depleted in impurity X into the exchanger in order to be cooled to a temperature below −50° C.; d) sending the gas stream depleted in impurity X and cooled to the cryogenic distillation unit wherein it is separated into at least a first stream and a second stream; e) introducing a portion of the first stream into the exchanger in order to be reheated to a temperature above −150° C. before being compressed in the compressor with a compression ratio greater than 1.2; f) sending the compressed first stream to the adsorption unit in order to regenerate one of the two adsorbers; with the compression in step e) leading to a temperature increase of the first stream of at least 20° C. and thus providing the heat input needed for the regeneration of at least one of the adsorbers.
 11. The purification process as claimed in claim 9, wherein the process comprises the following successive steps: a) cooling the feed gas stream in the exchanger to a temperature below −50° C.; b) sending the cooled gas stream to the adsorption unit wherein at least one impurity X is at least partly adsorbed so as to recover a first stream depleted in impurity X; c) compressing the gas stream depleted in impurity X in the compressor with a compression ratio greater than 1.2 before being cooled in the exchanger to a temperature below −50° C.; d) sending the gas stream depleted in impurity X, compressed and cooled to the cryogenic distillation unit wherein it is separated into at least a first stream and a second stream; e) introducing a portion of the first stream is introduced into the exchanger in order to be reheated to a temperature above −150° C.; f) sending the reheated first stream to the adsorption unit in order to regenerate at least one of the two adsorbers; with the compression in step c) leading to a temperature increase of the gas stream depleted in compound X of 20° C. and thus providing, indirectly via the exchanger, the heat input needed for the reheating of a portion of the first stream and therefore for the regeneration of at least one of the two adsorbers in step f).
 12. The process of claim 9, wherein the adsorbers comprise a monobed.
 13. The process of claim 9, wherein the feed gas stream is air and the impurity X is selected from the group consisting of H₂O, CO₂, N₂O, C_(n)H_(m), and NOx.
 14. The process of claim 9, wherein the feed gas stream comprises water and the process further comprises, before step a), a step of pre-purification of the feed gas stream that makes it possible to eliminate at least one portion of the water.
 15. The process of claim 14, wherein the pre-purification step is carried out by adsorption at ambient temperature.
 16. The process of claim 15, wherein the adsorption of the pre-purification step is carried out on a monobed of alumina, silica gel or molecular sieve type. 